The present invention relates to a paper feeding control apparatus for a serial printer in which a print head has a plurality of dot forming elements aligned in the paper-advancing direction and is scanned in the direction of paper width to print line by line, or a page printer using band memories.
A serial printer prints the entire surface of a sheet of recording paper by forming dots thereon while the print head is scanned in a direction of the paper width in such a way that the paper is advanced by one line each time the print head comes to the end of a line.
This type of printer is designed to print not only characters but also logotypes, enlarged characters, and graphics where a plurality of lines are used to print a single pattern. Thus, the number of steps of a pulse motor as well as the diameters of paper-advancing rollers and gear train ratio are selected so that the paper is accurately advanced a distance equal to N.times.P, where N is the number of dot forming elements of the print head and P is the pitch of dot forming elements.
However, the paper is not necessarily advanced in the precise manner intended due to dimensional errors of parts and assembly variations of the apparatus. Thus, in printing patterns aligned in the direction in which the paper is advanced, if the actual paper advancement per line is greater than the design value, then a blank A is left, as shown in FIG. 11A. Conversely, if the amount of actual paper advancement is smaller than the design value, then an overlap B is produced, as shown in FIG. 11B. This results in a poor printing quality.
Further, in a printer where a paper ejecting roller is designed to have a peripheral speed a little faster than paper advancement to ensure that the printed recording paper is properly ejected into a ejection tray, the recording paper is pulled by the ejecting roller, as a result of which the paper speed slightly shifts before and after passing this roller. This causes an error in a paper advancement for one line, producing blanks in a printed pattern.
Further, in a wire dot printer or the like, when the printer is used to print on a variety of recording sheets from very thick record media to very thin record media, e.g., when printing many sheets simultaneously, and printing a single sheet of paper, there may exist slight differences in paper advancement as the paper thickness is changed. Particularly, in the case where characters are printed on a ruled continuous paper such as slip paper, the printed characters may deviate from the ruled lines, and the end position of the printing may not be at a perforation of the paper for page section.
Still further, in a page printer using the band memory method where image data for one page is divided into a plurality of sections and stored in a memory, carriage error of a recording-paper advancing mechanism such as a photosensitive drum results in unprinted blanks or overlapped prints between bit map data divided into two adjacent bands.